This relates to the uniform coating of a substrate with a powder and more particularly to the uniform distribution of a powder through a slit to coat a substrate with a film having particular properties. For example, in the case of a glass substrate, the film might provide particular optical and electrical properties for the glass.
U.S. Pat. No. 4,230,271 and its corresponding French patent FR 2 427 141, which are incorporated herein by reference, describe an elongated nozzle for continuously distributing gas-entrained powders across a substrate. A stream of powder suspended in a gas is introduced into the nozzle through a multiplicity of conduits arranged in a row extending the length of the nozzle. The powder stream is then diluted with additional gas in an approximately rectangular homogenization chamber likewise extending the entire length of the nozzle. The gas-entrained powder is then directed through a passage that is oval in cross-section with walls that initially diverge, are then parallel and finally converge at a distribution slit above the substrate. Additional gas may be introduced into the powder stream by a gas supply located in the middle of the oval.
While this nozzle gives advantageous results, it is still quite susceptible to clogging and periodically requires cleaning to continue operating correctly. The downtime for cleaning results in lost production. Moreover, while such nozzles have been made with distribution slit lengths of 250 to 650 mm., the glass ribbons they are used to coat may be several meters wide, requiring the use of several identical nozzles placed end to end to coat the full width of the ribbon. In such circumstances, it is extremely difficult to balance the various nozzles so as to assure a regular distribution of powder coating over the entire width of the glass. It would be preferable to use a single nozzle with a distribution slit as long as the several meters of width of the glass ribbon; but it has been found that when the length of the nozzle is increased appreciably, the regularity of powder distribution is reduced. In particular, the track of the powder feed conduits can be discerned on the glass and clogging is faster.